Fermented milk is produced by fermenting milk, or milk or the like containing a non-fat milk solid content at the same level as or at a higher level than that of the milk, with lactic acid bacteria or yeast and then preparing the resulting product into paste or liquid or freezing the product. Fermented milk can be broadly classified into two types. One is pre-fermentation type while the other is post-fermentation type. The former is produced by disrupting fermented milk, on which fermentation and cooling have been completed in a tank prior to filling the milk in a container, and then filling the resulting fermented milk into an individual container for distribution. The latter is produced by filling a raw material mix for fermented milk (referred to as mix hereinafter) with a given amount of a starter added thereto in an individual container for distribution, such as paper container; fermenting the mix in a fermentation chamber until a lactic acid acidity reaches to a predetermined degree to solidify the mix into a pudding-like form; and then cooling the resulting product. Pre-fermentation is frequently used for producing fruit yogurt containing fruit and yogurt drinks, while post-fermentation is commonly used for producing yogurt of so-called hard type and plain type, and the like.
For any of the fermentation types, a fermentation step includes adding a starter to the sterilized mix at a given temperature for fermentation to a given acidity, and then cooling the resulting product to terminate the fermentation to obtain a product. The fermentation temperature and the fermentation time not only influence the production efficiency of the product but also highly influence the flavor and quality thereof. Therefore, the fermentation temperature and the fermentation time should appropriately be set, taking account of the influences thereon. In a case of intending to give mild flavor to the product, for example, the fermentation temperature is set to a lower temperature, which is accompanied by the fermentation time set to a longer time, and the like.
It is a problem herein that the fermentation time required until a given acidity is reached is prolonged far more than general, for example, in a case that the fermentation temperature is set lower so as to give mild flavor to the product; and the use of a longer fermentation time at a lower temperature causes a higher risk of microbial contamination, due to both the following factors: the temperature overlaps with a temperature zone where contaminating bacteria such as Escherichia coli (optimal growth temperature of 37° C.) and yeast (optimal growth temperature of 30° C.) actively proliferate and the induction period with a smaller pH change during fermentation is prolonged. As the method for reducing the risk, a method including sterilization under stronger conditions than general is proposed. However, the sterilization method has a relation with the physico-chemical properties of the final product. For example, in a case that sterilization is done under high temperature conditions such as UHT sterilization, the physico-chemical properties of fermented milk are so deteriorated that the mouth sensation of the final product is also deteriorated, disadvantageously.
Therefore, in the case of carrying out fermentation at a lower temperature for a longer time, examinations have been made about a sterilization process of a mix for fermented milk, without deterioration of the physico-chemical properties of the resulting fermented milk. For example, in the case that fermentation is to be done for a long time of 10 hours or more required for the raw materials for fermented milk to reach pH at 4.8, a sterilization process of the raw materials for fermented milk without deterioration of the physico-chemical properties of the final product has been reported, which comprises two-step sterilization. (Patent Document 1)
On the other hand, a process for producing fermented milk, by which the resulting fermented milk has not only fine texture and mild flavor but also such a tissue hardness as never found in fermented milk produced at a low temperature for a long time, so that the fermented milk consequently has acquired novel physico-chemical properties and mouth sensation, has been reported. The process includes carrying out low-temperature fermentation in conditions that a dissolved oxygen concentration in the mix for fermented milk is reduced and terminating fermentation for a short period of time. (Patent references 2 and 3).
The above process is used for producing hard yogurt of post-fermentation type and has an additional advantage that fermented milk with novel physico-chemical properties and texture as never found conventionally can be obtained by fermentation for a short time with a smaller risk of microbial contamination. However, this process is not suitable for producing pre-fermentation type where the tissue is damaged during a filling procedure, particularly for producing dessert-type fermented milk produced by adding and mixing fruit and the like.
Namely, since the process of Patent reference 2 or 3 is characteristic in that fermented milk having a hard tissue in comparison with that of fermented milk produced at a low temperature for a long period in the conventional art in addition to the characteristic texture can be obtained, the process is suitable for producing yogurt of post-fermentation type under no agitation during the production steps. Therefore, the process has not been completed for the purpose of producing products involving pre-fermentation in tanks and the like to be prepared on a prerequisite of the need of a procedure involving tissue damage, for example, yogurt of dessert type, such as fruit yogurt.
Patent document 1: JP-A-2003-125701
Patent document 2: Japanese Patent No. 3644505
Patent document 3: Japanese Patent No. 3666871